1. Field of the Invention
The invention relates to a hand-held laser distance measuring device with a pulse reflection mixing method, in particular a hand-held construction laser distance measuring device.
2. Description of the Prior Art
In the building industry, distances must be exactly determined with an accuracy of within a few millimeters at a range of up to several hundreds of meters distance. The hand-held laser distance measuring devices which are suitably constructed for this purpose and to which the present invention is directed use a pulse reflection mixing method of a modulated visible laser beam for measuring distance.
In EP610918B1, a short pulse train is used for distance measurement. After detection, this short pulse train excites an electronic resonator that is adapted to the pulse train frequency. The elevated signal of the resonator causes a laser to emit a new pulse train. This process is continuously repeated so that pulse cycles occur with a determined cycle frequency. The measured distance is determined by this cycle frequency.
DE3103567C2 introduces a method for direct measurement of the light pulse time-of-flight in which a measurement light pulse traveling over the measurement distance and a reference light pulse traveling over the reference distance are detected by the same photodetector. The detected measurement light pulse and reference light pulse start and stop a time measurement system, e.g., a fast counter. The measurement distance is determined definitively by means of direct and definitive measurement of the time difference between the detection of the reference light pulse and the detection of the measurement light pulse. The maximum repetition frequency of the light pulses is accordingly limited by the condition of the definitive determination of distance.
However, it is disadvantageous that measurement is impossible when the measurement distance corresponds to one half of the reference distance because the two pulses overlap. This problem can be solved by using a switchable reference distance in the form of a light-conducting fiber so that a different reference distance can be selected in case of overlapping pulses.
DE10112833C1 discloses a hand-held laser distance measuring device with a pulse reflection mixing method. The detection pulse train detected by the light detector or, in case of separate light detectors, the reference pulse train on the one hand and the measurement pulse train on the other hand are preferably directly subjected to direct mixing in the respective light detector followed by low-pass filtering. The direct mixing is controlled by a LO pulse train which is locally generated at the measurement point and whose duty factor is equal to, or approximately equal to, the duty factor of the measurement pulse train and whose repetition frequencies are selected so as to differ slightly.
Accordingly, the mixing pulse repetition frequency fM of the low-frequency pulse train corresponds to the amount of the difference between the pulse repetition frequency f of the transmission pulse train and measurement pulse train on the one hand and the pulse repetition frequency of the local oscillator pulse train fLO on the other hand. Therefore: fM=|f−fLO|. Like the high-frequency detection pulse train, the low-frequency mixing pulse train likewise comprises reference pulses and measurement pulses whose time delay is a measure of the distance. For further particulars, the person skilled in the art is referred to the above-cited document, whose disclosure is explicitly incorporated herein in its entirety.
In hand-held laser distance measuring devices of the type mentioned above using a pulse reflection mixing method, commercially available laser diodes emitting in the visible red wavelength range are used as laser sources. The emitted laser light is modulated by a series of very narrow spike pulses, hereinafter a transmitting pulse train, and bundled into a measurement laser beam by a collimating lens. Accordingly, this special hand-held laser distance measuring device with pulse reflection mixing requires a series of very narrow laser pulses with a usual width of between 60 ps and 80 ps as a transmitting pulse train.
The pulse repetition frequency f of the laser pulses ranging from 50 MHz to 200 MHz is very high compared to the pulse repetition frequency of several tens of kHz found in conventional hand-held pulse laser distance measuring devices, so that it is generally impossible to determine distances definitively at a range of up to several hundreds of meters distance with one measurement at a determined fixed pulse repetition frequency. Therefore, at least two measurements with two substantially different pulse repetition frequencies or differences of pulse repetition frequencies f1 and f2, and for very great distance ranges with high accuracy, even n different pulse repetition frequencies, are needed for a definitive determination of distance. Using an algorithm, a microcontroller determines the time differences τk, generally in a non-definitive manner, between the reference pulses and the measurement pulses of the low-frequency mixing pulses at different pulse repetition frequencies fk and, from the latter, determines the distance from the rangefinder to the light spot on the measurement object by means of the light velocity.
At certain distances with a time difference τk=0, an overlapping of the reference pulse and measurement pulse results due to a finite pulse width Δt. Since this renders measurement impossible, the microcontroller in this case chooses a slightly different pulse repetition frequency fki at which no overlapping occurs. It is disadvantageous that an increased distance measurement error must be taken into account even with a small τk because the two pulses influence one another due to the small distance.
In the following, the time difference between a reference pulse and a measurement pulse with respect to the period λ=1/fk is referred to as the relative time difference τk·fk and the pulse width with respect to the period λ=1/fk is referred to as the relative pulse width Δtλ·fk.
The object of the invention is to realize a hand-held laser distance measuring device with pulse reflection mixing having increased distance measuring accuracy. A further object is an algorithm for generating pulse repetition frequencies that are optimal with respect to distance measuring accuracy.